EPISODE: 013 - APPROVED DOCUMENT A - STRUCTURES - PART 2

BYTNAR - TALKS

EPISODE 013 - APPROVED DOCUMENT A - STRUCTURES - PART 2

This episode is for people who want to know more about Approved Document Part A - Structure.

You should like this episode if you ask yourself questions like:

• What does Approved Document A say about preventing disproportionate collapse in buildings?

• How do consequence classes (Class 1 to 3) impact building design requirements for robustness?

• What lessons were learned from the Ronan Point collapse that influenced current structural regulations?

• What are the stability tie requirements for Class 2a and Class 2b buildings?

• How is structural robustness tested when removing load-bearing elements in a building?

• What factors determine if a building falls into Class 3 for disproportionate collapse risk assessment?

• How are fire risks evaluated for structural safety in line with Approved Document A?

• Are basement stories exempt from disproportionate collapse requirements, and under what conditions?

• How do engineers balance safety and commercial considerations when designing for robustness against disproportionate collapse?

LET'S TALK ABOUT YOUR PROJECT

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This is Bytnar Talks: The Engineer Takes on Construction – Episode 13

Hi, I’m Piotr Bytnar.Each day, I help my clients plan and design building projects through Bytnar Limited, a consulting Chartered Structural Engineers practice.

My biggest passion—and the cornerstone on which I’ve built my business—is finding clever solutions for construction projects.

I am a Chartered Structural Engineer and a budding software developer, so you can rest assured that I will strive to talk about the best practices and the use of new technologies in the industry.

And if you’re embarking on a construction project—or are involved in planning, designing, and building the world around us—you’ll find this podcast useful.

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Approved Documents – Structure, Part Two

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Hi there, and welcome back to Bytnar Talks—your favourite podcast on all matters of architecture, engineering, and construction.

It is Thursday, the 18th of August 2024, and I’m back with you for the 13th episode, diving further into the Approved Documents.

It has been yet another busy week, and I had the pleasure and opportunity to visit BAE Systems down in Rochester.

It’s great to see how the biggest employer—or perhaps the second biggest, if you count the NHS—within our neighbourhood is delivering groundbreaking technology to the aerospace, maritime, and transportation industries.

It was an excellent example of how Six Sigma and Lean principles are being applied in a research-and-development-driven organisation.

Not to mention the fact that I got to use the simulators and fly some very nice jets, including fighter jets equipped with the latest onboard technology.

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Recap & Introduction

Last week in Episode 12, I talked about the first two parts of Requirement A – Structure.I discussed loading and ground movement, and decided to talk about disproportionate collapse separately in this episode.

I made that decision because loading and ground movement requirements are relatively straightforward to apply.However, the aspects of disproportionate collapse introduce further complexity to the considerations of structural safety.

All right, so let’s have a closer look at the subject.So, without further ado, let’s dive into the Disproportionate Collapse Requirements of the Approved Document.

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What Is Disproportionate Collapse?

Many good aspects of innovation are serendipitous in nature.And many aspects of law—and our conscience—are shaped in hindsight, often following a disaster.

Such is the origin of the disproportionate collapse requirement, which started with the Ronan Point collapse on 16 May 1968, killing four people.

If you don’t know what Ronan Point is:It was a 22-storey residential tower made of concrete panels assembled on site.Unfortunately, the construction technique was not robust enough—it lacked sufficient mechanical connection between the panels, which led to a catastrophic consequence.

A localized failure occurred—a gas explosion due to a leaking gas cooker connection pushed the walls out, allowing several storeys above to collapse, which in turn pulled the remaining lower storeys with them.

Interestingly enough, the lady who turned on the kettle did not die from the force of the explosion.It is said that she even took the cooker with her to her next place.Apparently, she didn’t take the kettle, as it had a quite a big dent in it.

And apparently, that dent gave forensic engineers the idea of what sort of pressure could have been exerted by the explosion—they settled on 34 kN/m².

So there you have it:A localized accident leading to large-scale damage that is not proportionate to its cause.That’s what the Disproportionate Collapse Requirement is all about.

This Episode’s Goal

In this episode, I’ll discuss:

• The considerations the Secretary of State draws our attention to

• And further expand on the consideration of risk and fire when it comes to building assessments

I’ll go section by section, giving you:

• Simple reasons behind the text

• And my commentary

All right, let’s get on to it.

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Approved Document A – Section 5: Disproportionate Collapse

Requirement A3, which is Disproportionate Collapse, is put in place to give us more robust structures by considering both the risk and the consequences of disproportionate collapse.

The Secretary of State provides an interpretation of this requirement in Section 5 of the Approved Document, titled:

"Reducing the Sensitivity of the Building to Disproportionate Collapse"

So, you see—it’s not just about elimination, but it’s about reducing the sensitivity.

As ever, with all of our law applications, with all our laws out there, we are dealing a lot with "reasonable" and "reducing risk", rather than being given some concrete advice—although, obviously, the Secretary of State tries to do his best here.

As we know, this is only guidance. And guidance becomes your argument, should you have a need to protect your specification and build.

But building officers generally have no other way of knowing differently, so the guidance is generally considered a minimum standard.

Yet, using it on its own does not mean the design and build complies with the requirement.The person who designs the project, the people who specify all the elements, and the people who build them, and then the people who operate the building—they need to make their own judgment and assure that they know what they’re doing.

So, what is this all about?

It’s about ensuring the building is robust enough, based on the consequence class it falls under.

Consequence classes, as these are called, are simple divisions of buildings into groups, or rather classes, for different and more stringent considerations.

This follows the general philosophy of the Approved Documents:Starting with the guidance, and ending up with systematic analysis of the building from first principles in different failure scenarios.

So, what are the classes?

What are we talking about when we refer to consequence classes of buildings under the consideration of robustness and disproportionate collapse?

There are basically four classes identified by the guidance—by the Approved Document. But in practice, there are five.

The fifth is "SARY", which means outside of the scope of all four classes identified within the guidance.

The Classes: 1 to 3 (with a split in 2)

The classes start from Class 1 and go to Class 3, with a split of Class 2 into 2A and 2B:

• 2A being the lower-risk group

• 2B being the upper-risk group

• And Class 3—which we Structural Engineers love like bees love honey

But let’s go from the lowest class first, shall we?

🏗️ Class 1 Buildings

These are buildings that can be built using a non-engineered approach, following the Approved Documents.

If you want to find out what the non-engineered approach is, I’d like to guide you to the previous episode, where I talked more about A1 and A2—considerations of the Approved Document.

🧱 Class 2A Buildings

This class considers buildings that can follow the requirements of Class 1 but need additional horizontal ties in the walls or need to make sure that floors are held down to the walls.

This requirement can be nearly read one-to-one with the reasons why Ronan Point collapsed.

• The floor was not tied down to the walls

• The walls were not tied to the floor

  
  

  So when the explosion happened, the floor was lifted, releasing the walls from their slots, which were then pushed out—and all that led to catastrophic collapse.

🧩 Class 2B Buildings

This class goes a step further.

It requires both horizontal and vertical ties between all loadbearing elements:

• Slab to beam

• Beam to column or wall

• And vice versa

So all structural elements carrying load must be tied together somehow.

Alternatively, to the above "tying" approach, the building may be checked for stability by:

• Interrupting the load path—removing one element like a wall, column, or beam

• Then seeing what happens

If no more than 15% of the storey (limited to 100 m²) gets destroyed, and the damage does not impact the floors immediately above or below, the building is considered robust by the Secretary of State.

But—if the damage extends over that limit, the removed loadbearing element should be strong enough to withstand the pressure exerted upon it by the explosion of a leaking gas cooker, which is taken as 34 kN/m².

If we can identify such a loadbearing element, it gets a nickname—it becomes known as a key element.

🏢 Class 3 Buildings

Now we get to Class 3.

Buildings in Class 3 need to be looked into with more care and understanding, taking into consideration:

• All probable and reasonable possibilities of building failure

• Including terrorist attacks and fire

So we now know what needs to be done—but what on Earth are these buildings?

Section 5.3: Examples of Class 3

In Section 5.3, the Approved Document elaborates on this.

For example:

• The length of the wall considered for potential removal depends on the material and method of construction

• Generally, this follows the principle of a panel one storey high and two and a quarter storeys wide for concrete or internal wall panels.

Otherwise, the panel between lateral supports would be sufficient to be taken into consideration for the tributary load path of that panel to the key element.

When considering an element for key element consideration, the explosion load should be considered both vertically and laterally—in both planes.Explosion doesn’t really happen only in one direction—trying to push walls—but it also tries to push ceilings and floors, so it should be taken into account in all directions.

Buildings Under Consideration

Let’s break down the building types under each consequence class.

🏠 Class 1 Buildings

• Houses, four stories or fewer

• Isolated buildings – meaning no immediate neighbours, or buildings that are at least one and a half times their height away from others

• In most cases, they will not impact more than a single large family if they collapse

🏘️ Class 2A Buildings (Lower-Risk Group)

• Houses up to five stories, if impacting only one family

• Hotels, flats, and offices up to four stories

• Three-story industrial buildings

• Three-story residential buildings, but with a limit of 2,000 m² floor area per storey

• Single-story education buildings

• Public buildings of max two stories, with a floor area limit of 2,000 m² per storey

🏢 Class 2B Buildings (Upper-Risk Group)

• Accommodation, educational, office, and retail buildings up to 15 stories

• Hospitals up to three stories

• Public buildings with up to 5,000 m² floor area

• Car parks up to six stories

🏟️ Class 3 Buildings

• Buildings that exceed the limits of the previous classes

• Includes grandstands with over 5,000 spectators

• Special designation buildings that deal with hazardous substances or processes

Interestingly, basement stories may be excluded from the calculations of storey count if they are constructed to a robust enough standard—at least Class 3 level. If they are, they can be excluded from the calculation.

For more complex buildings, risk may be evaluated and addressed by following the recommendations of Institution of Structural Engineers (IStructE) publications or other references recognised by the Secretary of State.

Risk Assessment & the Unthinkable

Truth be told—you need to understand the possible risks and find a way to either mitigate or eliminate them.

Did we think that the World Trade Center Twin Towers could be impacted by a Boeing 767?

No.

But they were.

Should we design all buildings for airplane impacts?

…I will leave you with that thought.

Similarly, with the impact of earthquakes—the Approved Document does not require typical buildings of Classes 1 to 2B to be assessed for this.But certainly, Class 3 buildings, or unusual buildings, should take this into account.

In reality, though, if you are in an earthquake-prone area, you will likely do this assessment for any building you design and allow for minimum ductility in the event of failure.

Bottom Line: Risk = Likelihood × Severity

The bottom line here is the relationship between risk and the severity of failure, and how that relates to:

• Harm to people

• (Though commercial risk is its own beast)

Here, we are talking about people’s safety—in and around the building.

And that leads us nicely to the topic of risk.

So, What is Risk in Disproportionate Collapse?

Risk in disproportionate collapse—as in any other scenario—is the combination of the likelihood and severity of any given event.

In Structural Engineering, we identify the risk and then we "ERIC the hell out of it."

ERIC = Eliminate, Reduce, Inform, Control

What does "ERIC the hell out of it" mean?

It means:

• Eliminate the risk

• Reduce the risk

• Inform the people exposed to the risk

• Control the risk

Or—if we cannot readily identify the issue, we take an approach of limiting the consequences of localised failures.This is exactly what is done in the design approaches for Classes 1 through 2B.

Engineering Perspective: It’s Not Just Years—It’s Wisdom

Any respectful engineer will look at the design from:

• Their own perspective

• And from the perspective of possible risk

But that perspective will be highly influenced by their wisdom—which means knowledge and discernment, not just years of experience.(Years are irrelevant in some circumstances.)

In many situations, it may not be prudent to just follow the expectations of the Approved Document.

Risk evaluation may be the better approach, especially when we’re modifying existing buildings.

A Key Omission: Fire

So, knowing what risk is and how we approach robustness, one of the biggest risks on any given project—and one that’s not even mentioned within the A3 requirements—is…

🔥 Fire.

The requirement of the Approved Document is written with localised structural scenarios in mind—mainly…

Would you like me to continue with the fire-related implications of disproportionate collapse, and how fire design intersects (or fails to intersect) with structural robustness?

Or I can summarise and prepare a CPD or downloadable version if you'd like that instead focuses on load path during accidental situations. One such accidental situation can be an event of fire—that, albeit not acting momentarily, usually impacts an entire compartment or a floor, and in this way, possibly many load-bearing elements.

The general approach to deal with fire in buildings is to isolate or reduce the temperature influence of the given fire scenario on a load-bearing element by providing protection or oversizing that element.

It would be unreasonable to build all types of buildings and then burn them down to the ground just to provide accurate guidance, so we rely on elemental fire resistance testing, and on some typical topology full-scale buildings—down to destruction fire testing.

It is, however, impossible to account for all circumstances and for all qualities of execution, to ascertain and make the kind of big optimizations the industry generally pushes for.

Another interesting fact is that materials are often tested to standards that may fail to recognize all aspects of the given material—like the propagation of heat waves within timber, which may lead to failure past the test finish, or simply not reflect the intensity and duration of a particular fire.

Naturally, temperature changes of structural elements can lead to localized values at the intersections with other elements—especially where those elements are restrained.

Fire risk should be looked at from the perspective of a fire engineer, who will analyze possible fire scenarios, and structural engineers, who will qualify the risk to the structure that such a scenario brings about.

Making sure the building remains robust under fire conditions may prove the most difficult design aspect of them all.

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Summary

So here you have it: Approved Document A, Requirement A3 – Disproportionate Collapse.

To sum up briefly, the requirement for disproportionate collapse aims to ensure buildings are robust enough to withstand various scenarios, considering the risk, the consequences of failure, and accounting for a certain level of uncertainty.

The Secretary of State provides guidance through Approved Documents, outlining different consequence classes—Classes 1 to 3—with increasing stringency as the number goes up:

• Class 1 buildings include small structures built to the approved documents.

• Class 2A requires additional ties for stability, addressing risks identified during forensic investigation of the Ronan Point collapse.

• Class 2B necessitates horizontal and vertical ties between load-bearing elements.

Alternatively, buildings can be tested by removing supporting elements.If the damage remains localized, the structure is deemed robust.

• Class 3 buildings undergo comprehensive risk assessments, including factors like terrorist attacks and fire.

• Specific criteria determine which buildings fall into each class, considering height, occupancy, and construction type.

• Basement stories may be exempt from calculations if built to a higher standard.

For more complex structures, alternative risk evaluation methods may be employed—balancing safety and commercial considerations.

Fire Safety

Fire safety is a crucial aspect—focusing on protecting load-bearing elements from temperature influence, through insulation or oversizing. However, testing standards may not fully capture real-world scenarios for an elemental fire approach.

Engineers analyze fire risks in line with the fire engineer’s evaluation of possible fire scenarios and their intensity, to ensure structural integrity during and post-fire.

Balancing risk, safety, and commercial interest, while focusing on protecting people’s safety in and around buildings, is paramount—and the reason for a robust structural design.

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Thank you for staying with me all the way till now.

I hope the added commentary and simple reasons behind the document’s paragraphs made your understanding of the matter that little bit better.

If you have any questions, reach out to me on LinkedIn or send me an email—I’m more than happy to help you out.

You can book a non-obligatory consultation on our website: www.bytnar.co.uk

We help our clients design and execute their dream homes or investments.

If your building is falling apart, we can also help investigate the reasons behind it and provide you with an appropriate strategy, design, and specifications for the repair.

Next week, I’ll start talking about Requirement B – Fire.

Thank you again for listening.Please voice up your opinions—I'm waiting for you on LinkedIn, and I want to hear from you.

Toodloo! 🎙️

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Piotr Bytnar BEng (Hons) MSc CEng MIStructE

Chartered Structural Engineer who deals with the Architecture of buildings. His Master's Studies led him to an in-depth understanding of risk and contract arrangements in construction as well as specialist knowledge in soil mechanics.

He and his team help homeowners and property developers to design and deliver construction projects reducing waste in time and the cost. He believes that the construction project is an iterative process that can be well managed and it is best managed if all the aspects of the project definition and management are dealt with in-house or coordinated by one organisation. His team works to all stages of RIBA and ISTRUCTE stages of work and enables contractors to deliver projects on-site providing risk evaluations, methodologies for execution of works and temporary works designs.

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